Cracking catalyst for hydrocarbon charges comprising an offretite, a zeolite and a matrix

ABSTRACT

The cracking catalyst of the invention contains: 
     (a) 20-95% by weight of at least one matrix, selected from the group consisting of alumina, silica, magnesia, clay, titanium oxide, zirconia, combinations of at least two of these compounds and alumina-boron oxide combinations, 
     (b) 1-60% by weight of at least one zeolite of open structure whose main dodecagonal channels have openings of at least 7 Angstroms, said zeolite of open structure being selected from the group formed of X, Y, L, omega and beta zeolites, 
     (c) 0.5-60% by weight of at least one offretite whose main dodecagonal channels have openings smaller than 7 Angstroms and which have a SiO 2  /Al 2  O 3  molar ratio from about 15 to about 500, the crystalline paramaters a and c of elementary mesh ranging respectively from 1.285 to 1.315 nm for a and from 0.748 to 0.757 nm for c and have a potassium content lower than 1.5% by weight.

This is a division of application Ser. No. 154,258, filed Feb. 10, 1988.

The present invention concerns a hydrocarbon charge conversion catalystcomprising a zeolite of open structure, associated with a stabilized anddealuminated offretite and an amorphous or insufficiently crystallizedmatrix.

The catalyst according to the present invention is particularly welladapted to the cracking of oil fractions in order to produce asubstantial amount of propylene, in particular to the cracking of heavyoil fractions.

The present invention also concerns a process for cracking oil charges,particularly heavy oil charges, in the presence of the above-definedcatalyst, as well as processes for manufacturing said catalyst.

BACKGROUND OF THE INVENTION

The introduction of the catalytic cracking process in the oil industryat the end of the thirties was a determinant progress with respect tothe prior techniques, by providing for highly improved yields ofhigh-grade motor gasoline. The various processes operating in fixed bed(e.g. HOUDRY processes) have been rapidly supplanted by moving bedprocesses and, particularly since the middle of the forties, by those ofthe fluid bed type (fluid catalytic cracking, in short FCC). At the verybeginning, the catalytic cracking processes were used nearly exclusivelyfor treating vacuum distillates (VD) of low sulfur content that arerelatively light (final boiling point lower than 540°-560° C.).

The cracking of these charges is generally conducted at about 500° C.under a total pressure close to atmospheric pressure and in the absenceof hydrogen. In these conditions the catalyst becomes quickly coveredwith coke and it is constantly necessary to regenerate it. In crackingprocesses of the fluid bed type (FCC) or of the moving bed type (such asTCC) the catalyst permanently circulates between the reaction zone whereit resides about a few seconds to several tens of seconds, and theregenerator where it is freed of coke by combustion between about 600°and 750° C. in the presence of diluted oxygen.

The fluid bed (FCC) units are now used more extensively than those ofthe moving bed type. The catalyst circulates therethrough in a fluidizedstate as particles of average diameter ranging from 50 to 70 microns,the granulometry of said powder ranging approximately from 20 to 100microns.

The catalysts used in the first FCC units were solids of high silicacontent obtained by acid treatment, either of natural clays or ofsynthetic silica-aluminas. The main progress in FCC achieved up to theend of the fifties was, in particular:

the use of the spray drying technology for preparing catalysts in theform of fine spherical particles more easily fluizidable and moreresistant to attrition than the powder obtained by crushing,

the synthesis of silica-aluminas, initially containing a high (about 85%by weight) silica and a low alumina (Lo-Al) proportion and then a highalumina (Hi-Al) content, with about 75% of SiO₂,

and various very substantial improvements concerning metallurgy andequipment design, particularly for regeneration.

It is only at the beginning sixties that a major advance took place inthe field of catalytic cracking, by the use of molecular sieves and moreparticularly of the zeolite of the faujasite structure, first in amoving bed process, then, a little later, in the FCC process. Thesezeolites, incorporated with a matrix mainly consisting of amorphoussilica-alumina and optionally containing variable proportions of clay,are characterized by cracking activities for hydrocarbons about 1000 to10000 times those of the first catalysts used. The availability on themarket of these new zeolite catalysts has completely changed thecracking process by the very substantial gain of activity and ofselectivity to gasoline and also by considerable modifications in theunit technology, particularly:

cracking in the riser (tube wherein the catalyst and the charge flowupwardly),

decrease of the contact time,

modification of the regeneration techniques.

These three points will be further examined hereinafter.

The X zeolite (faujasite structure) characterized by a SiO₂ /Al₂ O₃molar ratio from 2 to 3 has been first used. Exchanged to a large extentwith rare-earth ions, it is highly active and has a high thermal andhydrothermal resistance.

Towards the end of the sixties, this zeolite has been progressivelyreplaced by Y zeolite which tends to produce slightly less coke andwhose thermal and hydrothermal resistance was much improved. Presently,a major part of the proposed catalyst (probably more than 90%) containan Y zeolite exchanged with rare-earth ions and/or ammonium ions.

From the beginning of the sixties the oil industry began to suffer froma shortage of available crude oil, whereas the demand for gasoline ofhigh octane number was continuously increasing. Moreover, the availablesupply was progressively oriented towards heavier and heavier crudeoils. The treatment of the latter raised difficult problems for therefiner in view of their high content of catalyst poisons, particularlynitrogenous compounds and metal compounds (mainly nickel and vanadium),their exceptional Conradson carbon and overall asphaltene compoundvalues.

The necessity to treat heavier charges and other more recent problemssuch as: the progressive but general elimination of lead-containingadditives, the slow but substantial evolution in various countries ofthe demand for middle distillates (kerosene and gas-oil), have inducedthe refiners to make searches for finding improved catalysts whereby thefollowing objects can be met:

to obtain thermally and hydrothermally more stable catalysts, also moretolerant of metals,

to reduce the amount of coke formation at equal conversion rate,

to obtain a gasoline of higher octane number,

to improve the selectivity to middle distillates.

It is mostly desirable to reduce the production of light gasescomprising compounds of 1-4 carbon atoms and accordingly the catalystsare so designed as to limit the production of such light gases. But incertain countries, particularly in certain developing countries, thedemand for these products or for some of them, particularly propylene,may be high. The catalytic cracking process may meet to a certain extentsuch a demand, provided that the catalyst be particularly adapted tosaid production.

An efficient way for adapting the catalyst consists of adding to theconventional catalyst masses an active agent having the two followingproperties of:

1. Cracking heavy molecules with a good selectivity to C₃ hydrocarbons,particularly propylene,

2. Sufficiently resisting to the severe conditions of steam partialpressure and of temperature prevailing in a regenerator of an industrialcracking unit.

As a matter of fact, in view of the tendency of the present charges toproduce more and more coke which deposits onto the catalysts, and of thehigher sensitivity to coke of zeolite performance, the present researchhas as an object not only finding catalysts less selective to coke, butalso to further catalyst regeneration in order to reduce to a minimumthe coke amount at the end of the combustion. In many processes, this isachieved by increasing the regenerator temperature.

Consequently, the regenerator is subjected to high steam partialpressures, ranging from 0.2 to 1 bar (1 bar=0.1 MPa), and to localtemperatures at the catalyst level of 750°-850° C. or even 900° C., fora few tens of seconds to a few minutes. In these conditions the zeolite,which is the main active agent of the catalyst, may rapidly lose a largepart of its activity due to the irreversible degradation of itsstructure. In spite of various artful techniques developed during thelast years for limiting the regenerator temperature (addition of coilsto remove heat by producing steam or intermediary cooling of thecatalyst) or for limiting the steam content at high temperature (atechnique using two regenerators as that used in the R2R process ofTOTAL-IFP), the zeolite present in the cracking catalyst mustnecessarily have an excellent thermal and hydrothermal stability.

SUMMARY OF THE INVENTION

Among the zeolites which might be used to obtain a catalyst having theabove-mentioned required properties, it has surprisingly been discoveredthat a stabilized and dealuminated offretite provides a catalyst clearlysuperior to that obtained for example with one of the following zeoliteswhich, a priori, could have been considered as possibly interesting andwhich have been surveyed during the work which has led to the presentinvention:

erionite of T erionite: these zeolites have a good selectivity forproducing C₃ hydrocarbons, but their hydrothermal stability is clearlynot so good as that of offretite and, in addition, they suffer from thedisadvantage of having too closed a pore structure, making theminefficient for the conversion of certain bulky hydrocarbon molecules,such for example as isoparaffins, naphthenes and alkylaromatics.

ZSM5: this zeolite, hydrothermally very stable, is insufficientlyselective for the production of C₃ hydrocarbons, as shown in one of theexamples given hereinafter.

Ferrierite: this zeolite is hydrothermally very stable, but its porestructure is clogged at high temperature, in the presence of steam, byextensive extraction of aluminum atoms from the aluminosilicatestructure. As a matter of fact, these extracts are housed in themicropores, thus blocking the access to the molecules, and they cannotbe substantially removed by chelating or acid treatments. Moreover, whenit still has a slight activity, ferrierite leads to a too largeproportion of C₁ or C₂ hydrocarbons to be of interest.

Mordenite: this zeolite is hydrothermally very stable and easy tostabilize and to dealuminate, but its selectivity for the production ofC₃ hydrocarbons is clearly insufficient.

The stabilized and dealuminated offretite used for manufacturing thecatalyst according to the invention is obtained by modifying treatmentsproviding for the possibility to adjust at will the aluminum and siliconcomposition of its aluminosilicate structure. Its manufacture has beendisclosed in European patent 190,949 whose description is incorporatedherein by way of reference.

The modifying treatments constitute one of the rare means, other thansynthesis, for obtaining zeolites of highly increased silica content.

Zeolites pertaining to other groups than pentasils or related groups(mordenite group) such as Y zeolites, omega zeolite (or mazzite),erionite, offretite, chabazite, etc.. can only be obtained exceptionallywith SiO₂ /Al₂ O₃ ratios higher than 12-15, by hydrothermal synthesis.

The modifying treatments for substantially changing the composition ofthe aluminosilicate structure, are highly recommended for adjusting theacid properties of zeolites and improving their performance.

According to the present invention, zeolites of the modified acidoffretite type have been used to prepare a cracking catalyst forhydrocarbon charges whereby an improved selectivity for the productionof C₃ hydrocarbons, particularly propylene, may be obtained.

The offretite is a natural or synthetic zeolite pertaining to thechabazite group. Its structure was long considered as identical to thatof erionite, zeolite of the same family, due to their similar X-raydiffraction spectra (HEY M. H. & FEJER E. E, Min. Mag. 33, 66, 1962).However, these two structures are different:

1--the hexagonal mesh of the offretite has a size along c axis which isone half of that of erionite (BENNET J. M. & GARD J. A., Nature 214,1005, 1967), and also the odd lines 1 of erionite X-ray diffractionspectra are absent from the offretite spectra (GARD J. A. & TAIT J. M.,Molecular Sieve Zeolites-1, Advan. Chem.,Ser. 101, 230, 1971);

2--the piling sequences of the two zeolites are different (WHYTE T. E.Jr., WU E. L., KERR G. T. & VENUTO P. B., J. Catal. 20, 88, 1971). Thusthe offretite has a much more open structure than erionite. Pilingdefects may occur in these structures, giving rise to the formation of Terrionite, which is a zeolite of offretite structure with piling defectsof the erionite type.

The offretite structure has been specified by many authors. This zeoliteis formed of two types of columns: columns of 14 face-gmelinite cages ofabout 3.6 Angstrom diameter and columns of 11 face-concrenite cagesalternating with hexagonal prisms. These columns are oriented along thec axis and lead to the formation of two types of channels largedodecagonal channels parallel to the c axis, of diameter ranging from 6to 7 Angstroms (1 Angstrom=10⁻¹⁰ m) and octagonal channels perpendicularto the c axis.

Offretite is mostly synthesized in the presence oftetramethylammonium(CH₃)₄ N⁺ - or TMA- and potassium K⁺ ions. The molarSiO₂ /Al₂ O₃ ratios generally range from 4 to 10. From theabove-described structure, four locations are possible for thesecations: the hexagonal prisms, the cancrinite cages, the gmelinite cagesand the large channels. The distribution of these cations is as follows(AIELLO R., BARRER R. M., DAVIES J. A. & KERR I. S., Trans. Faraday Soc.66, 1610, 1970; AIELLE R. & BARRER R. M., Chem. Soc. (A), 1470,1970):the potassium ions are located in the hexagonal prisms, the concrenitecages and eventually in the large channels; the TMA ions, more bulky,are located in the gmelinite cages and in the large channels. Thisdistribution results in the following theoretical exchanges: TMA or K⁺ions located in the large channels are accessible, and exchangeable,whereas the TMA cations located in the gmelinite cages are "trapped",hence unexchangeable; the potassium ions located in the concrenite cagesare a priori accessible, hence exchangeable; on the contrary, thoselocated in the hexagonal prisms are inaccessible, hence unexchangeable.

Presently, the direct synthesis of offretite of high silica content isnot possible. Modifications of this solid, i.e. dealumination, must beused.

Dealuminated offretite

It is possible, from a K-TMA offretite of silica-to-alumina molar ratiofrom 4 to 10 (or 6 to 10), whose X-ray diffraction diagram andcrystalline parameters are those reported in table A, to obtain anoffretite of high silica content, whose silica-to-alumina molar ratio ispreferably much higher than 10, by performing cycles of thermaltreatments - liquid phase treatments. These thermal treatments areconducted at temperatures from 400° to 900° C. The liquid phasetreatments consist either of cation exchanges or of acid etchings. Theprocess must comprise at least one ion exchange and at least one acidetching. Cation exchanges are performed in at least one solution ofionizable ammonium salt, at a temperature from 0° to 10° C. Acidetchings are performed with inorganic acids such as hydrochloric acid,nitric acid, hydrobromic acid, perchloric acid or organic acids suchacetic acid or benzoic acid, for example, at temperatures ranging from0° to 150° C. The obtained product has a diffraction spectrumcorresponding to that of offretite with crystalline parameters of thefollowing size: a from 1.285 to 1.315 nm and c from 0.748 to 0.757 nm (1nm=10⁻⁹ m), a benzene absorption capacity higher than 5% and preferablyhigher than 6% (conditions specified hereinafter), a secondarymicroporosity, measured by the BJH method, from 3 to 5 nm andcorresponding to about 5-50% of the zeolite total pore volume (the BJHmethod is explained hereinafter).

                  TABLE A    ______________________________________    2 Theta d (nm)   Intensity                              2 theta                                     d (nm) Intensity    ______________________________________    7.65    1.153    100      27.00  0.330  19    11.70   0.755    14       28.35  0.315  26    13.40   0.661    29       30.55  0.292   4    14.05   0.629     8       31.40  0.284  100    15.45   0.573    16       33.50  0.267  13    19.45   0.456    27    20.55   0.432    41    23.36   0.380    31    23.72   0.375    78    24.85   0.358    41    ______________________________________

X-ray diffraction diagram of a K-TMA offretite (9.9% by weight K⁺)having a SiO₂ /Al₂ O₃ molar ratio of 8.

Crystalline parameters: a=1.322 nm; c=0.752 nm

Characterization The obtained offretite of high silica content has beencharacterized by the following techniques:

X-ray diffraction

The apparatus and methods are disclosed in the European patent 190 949to the applicant.

Water absorption

Each sample is activitated by preheating at 425° C. for 16 hours under apressure of 5×10⁻³ mmHg in a conventional apparatus of Mc Bain type.

Then the temperature of the sample is adjusted to the desired value (25°C.) and the sample is contacted with steam at the desired pressure (P/P₀=0.1).

Benzene adsorption

This test is performed with the same type of apparatus and the same typeof activation than for measuring water adsorption. The sampletemperature is set at 25° C. and the benzene pressure at about 70 bars.The adsorption time is set at 4 hours.

Microporosity

The microporosity is determined by the BJH technique (BARRET, JOYNER,HALENDA, J. Am. Chem. Soc. 73, 373 (1951)) based on digital processingof the nitrogen desorption isotherm; the total pore volume is consideredat a nitrogen pressure such that P/P_(o) =0.9, P being the nitrogenpartial pressure in the measurement conditions and P_(o) the nitrogensaturating vapor pressure at the temperature of measurement.

The potassium, sodium, silicium and aluminum determinations have beenperformed by chemical analysis.

Obtainment of the product

The starting product is a synthetic offretite containing TMA(tetramethammonium) and K⁺ ions (K⁺ being close to 10% by weight) orTMA, K⁺ and Na⁺ (0.5<K/Na+K<0.8) whose silica-to-alumina molar ratioranges from 4 to 10.

The offretite according to the present invention, having asilica-to-alumina molar ratio of at least 15 has been obtained bysubjecting an offretite, partially exchanged by the conventionaltechnique of cations exchanges of the prior art, to at least one cycle,preferably at least two successive cycles of thermal treaments-liquidphase treatments, as above-described.

The type of operations performed on an offretite containing TMA andpotassium ions (9.9% by weight) is as follows:

(1) According to the prior art techniques a product of potassium contentfrom 1.5 to 3% by weight is first prepared, a potassium content of lessthan 1.5% by weight being not possible to obtain by this technique. Themethod comprises the steps of:

Removing TMA cations by roasting in air, at flow rates from 0.01 to 100l/h/g, preferably from 0.5 to 10 l/h/g and at temperatures preferablyfrom 450° to 650° C. for more than half an hour.

Performing at least 3 cation exchanges, according to the conventionaltechnique, at temperatures from 0° to 150° C., with a ionizable ammoniumsalt such as nitrate, sulfate, chloride, ammonium acetate, of molarityhigher than 0.1 M, preferably higher than 1 M and more generally lowerthan 3 M; these exchanges are performed with a ratio of solution volumeto the weight of dry solid (V/P) higher than 2, preferably higher than3, generally lower than 20 and often even lower than 10.

The solid obtained by this series of cation exchanges contains about1.5-3% by weight of potassium.

(2) From the product obtained by the above-described prior art method,an offretite of increased silica content, of silica-to-alumina molarratio higher than 10, is prepared by at least one cycle of the followingoperations:

Roasting in air, preferably containing 5-100% of steam, at flow ratesfrom 0.01 to 100 l/h/g, preferably from 0.5 to 10 l/h/g, at temperaturesranging from 400° to 900° C., preferably from 500° to 800° C., for atleast 0.5 hour, preferably more than one hour.

The roasting in moist air may be replaced by a roasting in a staticatmosphere, i.e. in the absence of any gas flow, the moistnessoriginating from the product itself. This procedure may be calledself-steaming or roasting in confined atmosphere.

At least one cation exchange in the above-described conditions or atleast one acid etching at temperatures from 0° to 150° C., with aninorganic acid such as hydrochloric acid, nitric acid, hydrobromic acid,sulfuric acid, perchloric acid, or with an organic acid such as aceticacid or benzoic acid (of normality higher than 0.1 N and preferably from0.5 to 5 N), with a V/P ratio higher than 2 and preferably higher than3, generally lower than 50 and often lower than 10, for periods longerthan 10 minutes.

The process generally comprises at least one cation exchange and atleast one acid exchange.

The acid etching may be performed under controlled conditions, accordingto the following procedure: the solid is first suspended into distilledwater at a temperature from 0° to 150° C.; then the necessarytheoretical acid amount for extracting the desired aluminum amount (3 H⁺protons per aluminum) is added dropwise. Preferably H⁺ ions are lessthan the total aluminum amount contained in the solid. After acidaddition, stirring is continued for more than half an hour. A modifiedoffretite according to the invention, of a SiO₂ /Al₂ O₃ molar ratiohigher than 15 is thus obtained from an offretite having a SiO₂ /Al₂ O₃molar ratio ranging from 4 to 10, containing 1.5-3% potassium, by thefollowing operation cycle, performed after previous cation exchange:

roasting in moist air or self-steaming,

treatment with an acid,

and/or further by the following operation cycle:

roasting in moist air or self-steaming,

cation exchange with ammonium ions,

roasting in moist air or self-steaming,

treatment with an acid.

It is hence possible that the first liquid phase treatment be an acidetching. However, the latter must be conducted with much care, i.e. at anot too low p_(H) value and a not too high temperature in order topreserve the solid crystallinity. Generally, particularly for startingoffretites of relatively low SiO₂ /Al₂ O₃ molar ratio, the first liquidphase treatment is preferably a cation exchange. It is also possible toperform a liquid phase treatment consisting of a mixed cationexchange/acid etching treatment by admixing an acid with an ammoniumsalt.

The optimization of the method, the selection of the one or more firstliquid phase treatments, the number of roasting-liquid phase treatmentcycles to perform, depend on the initial SiO₂ /Al₂ O₃ molar ratio,acting on the product stability, and on the final SiO₂ /Al₂ O₃ molarratio. Thus the number of operations required for obtaining a given SiO₂/Al₂ O₃ ratio will be higher as the initial ratio is low and the finalratio high.

The acid offretites of increased silica content according to the presentinvention are characterized by the following properties:

SiO₂ /Al₂ O₃ molar ratio higher than about 15 and lower than about 500,

crystalline parameters a and c such that a ranges from about 1.285 to1.315 nm and c from about 0.748 to about 0.757 nm,

potassium content lower than 1.5% by weight,

preferably with, in addition:

benzene adsorption capacity at 25° C., under a partial pressure of 70torrs (133.32×70 Pa) higher than 5% by weight,

existence of a secondary microporosity, as measured by the BJH method,within 3-5 nm and corresponding to about 5-50% of the zeolite total porevolume, which is determined by nitrogen adsorption for a P/P_(o) ratioof 0.9.

It has been observed that the more stable catalysts contained offretitesof highly increased silica content. More precisely these offretites havethe following characteristics:

SiO₂ Al₂ O₃ molar ratio higher than about 20 and preferably higher thanabout 30 (particularly from 20 to 400 and more particularly from 30 to300),

crystalline parameters a and c such that a ranges from about 1.290 toabout 1.310 nm and preferably from about 1.294 to about 1.300 nm and cranges from about 0.750 to about 0.755 nm,

potassium content lower than 1% by weight, preferably lower than 0.5% byweight,

preferably with, in addition:

benzene adsorption capacity at 25° C. under a partial pressure of 70torrs, higher than 5 and preferably higher than 6% by weight,

water adsorption capacity, at 25° C. for a P/P_(o) ratio of 0.1, lowerthan 18% and preferably lower than 13% by weight,

existence of a secondary microporosity, measured by the BJH method,within 3-5 nm and corresponding to about 5-50% of the zeolite total porevolume.

Obviously, the modifying treatments just described for offretite may beapplied to all the structures of erionite-offretite type andparticularly to ZSM-34 zeolite.

The catalyst according to the invention contains:

(a) 20-95%, preferably 30-80% and advantageously 50-80% by weight of atleast one matrix (constituent A),

(b) 1-60%, preferably 4-50% and more advantageously 10-40% by weight ofat least one zeolite of open structure other than offretite (constituentB); according to the present invention, the term zeolite of openstructure designates a zeolite whose main dodecagonal channels have anopening of such a size that it is equivalent to a circular opening of atleast 7 Angstrom diameter,

(c) 0.5-60%, preferably 1-40% and more advantageously 2-30% of at leastone offretite whose dodecagonal channels have an opening lower than 7Angstroms (constituent C), having a SiO₂ /Al₂ O₃ molar ratio from about15 to about 500, crystalline parameters of elementary mesh a and c suchthat a ranges from 1.285 to 1.315 nm and c from 0.748 to 0.757 nm and apotassium content lower than 1.5% by weight, the total alkali metalcontent being preferably lower than 1.5% by weight.

The sum of the percentages by weight of constituents A, B and C of thecatalysts is always 100%.

Constituent A of the catalyst according to the invention comprises atleast one matrix generally selected from the group formed of alumina,silica, magnesia, clay, titanium oxyde, zirconia, a combination of atleast two of said compounds and alumina-boron oxide combinations.

Examples of combinations of at least two compounds of the above groupare silica-alumina and silica-magnesia.

Examples of preferred constituent A are silica, alumina, magnesia,silica-alumina and silica-magnesia mixtures.

Constituent B of the catalyst according to the invention comprises atleast one zeolite of open structure having dodecagonal channels of atleast 7 Angstrom opening, generally selected from the group formed of X,Y, L, omega and beta zeolites. Zeolites of faujasite structure,particularly Y zeolite, preferably stabilized, currently calledultrastable or USY, or zeolites of increased silica content such asLZ210 zeolite disclosed in U.S. Pat. Nos. 4 403 023 and 4 534 853 and inEuropean patents 139 291 and 124 120, are preferably used, either atleast partially exchanged with cations of alkaline-earth mainly cationsof rare-earth metals having an atomic number ranging from 57 to 71inclusive, or in hydrogen form.

Constituent C of the catalyst according to the invention (different fromconstituent B) comprises at least one offretite selected from theabove-defined offretites. The offretite may used in a hydrogen form,hence pratically containing no metal except the very small amount ofalkali metal cations (mainly potassium) which are residual cationsoriginating from the zeolite synthetis. The offretite may also be atleast partly exchanged with multivalent metal cations; a part of thecation sites of the alumino-silicate structure is then occupied by thesecations; such cations are for example cations of alkaline-earth metals,preferably cations of rare-earth metals of atomic number from 54 to 71inclusive, more particularly lanthanum; these cations being destined toblock the structural evolution of the specific offretite used which isliable to occur in the severe operating conditions of the crackingindustrial unit regenerator.

The modified offretite, used for the preparation of the catalystaccording to the invention, such as above-described, is an offretitewhose alumino-silicate structure consists exclusively of aluminum atomsand silicon atoms. However, it is also possible to prepare the catalystaccording to the invention from a modified offretite, such asabove-described, wherein a part of the aluminum and/or silicon of thealuminosilicate structure is replaced, at the end of the synthesis, byother elements, metals or metalloids, such for example as B, P, Ti, V,Cr, Fe, Mn, Ga, Ge and Zr.

The catalyst of the present invention may be prepared by any method ofthe prior art.

Thus the catalyst may be obtained by simultaneous incorporation ofoffretite and zeolite of open structure with a matrix, according toconventional methods of manufacturing zeolite-containing crackingcatalysts. The catalyst may also be obtained by mechanical mixing of aproduct comprising a matrix and a zeolite of open structure, such forexample as a Y zeolite, and a product comprising the above-describedmodified offretite which is for example a mixture of said offretite witha matrix; the offretite-matrix mixture generally contains 1-90% byweight and preferably 5-60% by weight of offretite in proportion to thetotal mixture weight.

The mixture matrix-zeolite of open structure used for preparing thecatalyst according to the invention is generally a conventional crackingcatalyst according to the prior art (for example a commercial catalyst);the above-described modified offretite used to manufacture the catalystof the invention may thus be considered as an additive which may be usedas such, in view of being admixed with the above-defined conventionalcracking catalyst, or may be previously incorporated with a matrix, theassembly formed by the matrix and the modified offretite,, thusconstituting the additive which is admixed with the above-definedconventional cracking catalyst, for example, after adequate shaping, bymechanical mixing of particles containing the modidied offretite withparticles of conventional cracking catalyst.

The general conditions of the catalytic cracking reactions are wellknown (see for example U.S. Pat. Nos. 3,293,192, 3,449,070, 4,415,438,3,518,051 and 3,607,043) and need not to be repeated here.

In order to produce the greatest possible propylene amount, it may beadvantageous to slightly increase the cracking temperature, for exampleby about 5°-30° C. Generally the catalyst according to the invention ishowever sufficiently active to make this temperature increaseunnecessary. The other cracking conditions are the same as those of theprior art.

According to an alternative embodiement of the invention, theabove-described catalyst may be subjected to modifications for a betteradaptation to catalytic cracking, in order to increase the octane numberof the gasoline produced by cracking.

Cracking processes are of common use in the oil industry. They consistof splitting hydrocarbon molecules of high molecular weight and highvolume point to smaller molecules boiling in a lower temperature range,convenient for the desired use.

The so-called Fluid Catalytic Cracking, in short FCC, process has beenbriefly described above. In this type of process, the hydrocarbon chargeis vaporized by contact at high temperature with a cracking catalystmaintained in suspension in the charge vapors. After the desiredmolecular weight has been reached by cracking, acccompanied with acorresponding decrease of the boiling point, the catalyst is separatedfrom the obtained product, stripped, regenerated by combustion of theformed coke and then again contacted with the charge to be cracked.

New FCC processes comprise the use of two regeneration zoneswherethrough circulates the used catalyst.

The charges to be cracked are usually introduced into the reaction zoneat a temperature generally from 480° C. to 540° C., under a relativepressure of 0.7-3.5 bars, whereas the temperature of the regeneratedcatalyst supplied to said zone may be of about 600°-950° C.

The catalyst is fed at the bottom of the riser in an amount adjusted forexample by the opening or closing of a valve. The catalyst particles arethen conditioned and speeded up towards the riser top, by injecting agas at the bottom thereof. This injection is performed by means of afluid distributor. The charge to be cracked is introduced at an upperlevel and at least partly vaporized by means of an appropriate device inthe dense flow of catalyst particles.

The riser top opens into an enclosure, for example concentric thereto,wherein the cracked charge is separated and the used catalyst isstripped. The catalyst is separated from the effluent, driven by acyclone system and then purified.

The hydrocarbon charges to be fed to units of the above-described typemay contain hydrocarbons having boiling points from 200° to 550° C. ormore and their density may vary from 30° to 35° API. These charges maybe heavy charges containing hydrocarbons whose boiling point may reach750° C. or more and whose density may range from 10° to 35° API, or evenfrom 0° to 25° API.

Examples of such charges are those having final boiling points of about400° C., such as vacuum gas-oil and also heavier hydrocarbon oils suchas crude and/or stabilized oils, and straight-run or vacuum residues.These charges are optionally subjected to a previous treatment such forexample as a hydrotreatment in the presence of catalysts, for example ofcobalt-molybdenum or nickel-molybdenum type.

Said charges are optionally diluted with hydrocarbon cuts previouslysubjected to cracking, which are recycled, as for example light cycleoil (L.C.O.) and/or heavy cycle oil (H.C.O.) and/or the unconvertedheavier fraction generally boiling above 500° or 550° C., commonlycalled slurry. According to a preferred embodiment, these charges areavailable and preheated within a temperature range from 300° to 450° C.before their treatment.

The improved catalyst used according to the present invention providesfor an improved flexibility in the structure of the product yields andthe quality of the gasoline cut, i.e. the Research and Motor octanenumbers of the latter.

It must be recalled, as a general rule, that, in the domain of catalyticcracking the operation of the process depends in particular on thenature of the desired hydrocarbon effluents in relation with theprevailing refining objects. Thus, by catalytic cracking of an oil, itis generally possible to obtain:

light gases (hydrogen, C₁ -C₂ hydrocarbons),

propylene,

propane (C₃),

saturated C₄ and iso-C₄ hydrocarbons,

C₄ unsaturated hydrocarbons,

gasolines,

light cycle oil or light diluent (L.C.O.),

a heavy cycle oil or heavy diluent (H.C.O.),

a residue or slurry, generally freed of catalyst particles, forobtaining a clarified oil (C.O.) or a decanted oil (D.O.).

It may be desired to obtain, by catalytic cracking, an increased amountof liquified petroleum gases (C₃ -C₄ or LPG) and more particularly anincreased amount of propylene or even butenes and/or isobutane. On theother hand, it is well known that the general tendency is to reduce theamount of lead-containing additives in gasolines used as motor-fuels,thus requiring the production, particularly by catalytic cracking, ofgasolines having improved Clear and Research octane numbers.Accordingly, it may be convenient either to substantially increase theproduction of unsaturated C₃ (propylene) without increasing theproduction of C₄ hydrocarbons and of saturated dry gases (H₂, C₁, C₂),or to substantially improve the production of unsaturated C₃ (propylene)and iso-C₄ without noticeably increasing the production of saturated drygases (H₂, C₁, C₂), or to substantially increase the production ofunsaturated C₃ (propylene) and optionally of C₄, particularlyunsaturated C₄, without increasing the production of saturated dry gases(H₂, C₁, C₂). In all of these three options it is more often desired tofurther obtain a maximum production of gasoline of particularly highoctane number. The two last options and the octane number increase arethe object of the present application and may be obtained by using a newspecific catalytic cracking catalyst.

Several of these objects, particularly a noticeable improvement of thegasoline quality and an increase of the yields to propylene, butenes andisobutane, can be simultaneously obtained with said specifically adaptedcatalyst.

The process according to the invention consists of adding to the mainconstituent, which contains at least one zeolite of faujasite structure(X or Y zeolite) a small amount of a zeolite whose main pores have asmaller opening than that of faujasite, smaller than 7 Angstroms and forexample ranging from 0.60 to 0.68 nm.

Among zeolites adapted to form a catalyst which improves the yields toC₃ and C₄ light products, particularly the yields to propylene, butenesand isobutane, and overall to increase very substantially the Researchand Motor octane numbers (RON and MON) of gasoline, it has beensurprisingly discovered that zeolites of the erionite family, such aserionite, offretite or zeolites related thereto, such as ZSM-34 or AG2,N-O, ZKU zeolites or still zeolites formed of erionite and offretite (Terionite for example) mixed crystals have these properties. Thus, theyprovide for clearly increased yields of propylene, butenes and isobutaneand gasoline octane numbers clearly higher than those obtained by usingconventional cracking zeolite catalyst, particularly when used asadditive to the latter. Among said zeolites of the erionite family, ithas been further discovered that the stabilized and dealuminatedoffretite,s particularly characterized by a silica-to-alumina molarratio of at least 15, provide particularly efficient catalysts. One ofthe zeolites preferred according to the invention is a stabilized anddealuminated offretite. It is obtained by means of modifying treatmentsfor adjusting at will the aluminum and silicium contents of itsaluminosilicate structure. Its preparation has been disclosed in theEuropean patent application 190 949.

According to the present invention, zeolites of the modified acidoffretite type have been used to prepare a catalyst for crackinghydrocarbon charges, providing for an improved quality (higher gasolineoctane numbers) and an improved production of C₃ and C₄ hydrocarbons,particularly of propylene, butenes and isobutane.

The offretite has been described above.

The specific catalyst used according to the invention contains a mixtureof matrix, of zeolite with open structure and of zeolite of the erionitefamily. Such a catalyst may be prepared by any method known in the art,as above indicated.

As already mentioned, the mixture of matrix and zeolite of openstructure used for preparing the catalyst according to the invention isgenerally a conventional cracking catalyst of the prior art (for examplea commercial catalyst). The zeolite may be a X, beta, omega zeolite,mainly a Y zeolite, more particularly an ultrastable Y zeolite, enrichedfor example with at least one metal of the rare-earth family, or a new Yzeolite enriched with silica by chemical treatments, called LZ210 anddisclosed in particular in U.S. Pat. Nos. 4 503 023 and 4 534 853 and inEuropean patents 139 291 and 124 120. The zeolite of the erionitefamily, for example an offretite, used to manufacture the catalyst ofthe invention, may then be considered as an additive admixed to theabove-defined conventional cracking catalyst, or may be previouslyincorporated with the matrix, the matrix-offretite assembly thus formingthe additive which is admixed to the above-defined conventional crackingcatalyst, after adequate shaping, by mechanical mixing of particlescontaining for example offretite with conventional cracking catalystparticles.

Although all the zeolites of the erionite family, particularlyoffretite, ZSM-34, AG2, N-O or ZKU zeolites or still T erionite, may beconvenient for the present invention, it has been discovered that themost efficient catalysts for selectively producing propylene containedoffretites of highly increased silica content. More precisely, theseoffretites have the following characteristics:

SiO₂ /Al₂ O₃ molar ratio higher than about 15 and preferably higher thanabout 20 (particularly from 15 to 500, and more particularly from 20 to300),

crystalline parameter a ranging from about 1.285 to 1.315 nm, preferablyfrom about 1.290 to about 1.310 nm, and crystalline parameter c fromabout 0.748 to about 0.757 nm,

potassium content lower than 1.5% by weight and preferably lower than0.5% by weight, and in addition:

nitrogen adsorption capacity, at 77 K and at a P/P_(o) ratio of 0.19,higher than 0.15 cc liquid per gram and preferably higher than 0.20 ccliquid per gram,

cyclohexane adsorption capacity, at 25° C. and at a P/P_(o) ratio of0.25, higher than 3 and preferably higher than 4% by weight,

water adsorption capacity, at 25° C. for a P/P_(o) ratio of 0.1, lowerthan 15% and preferably lower than about 10% by weight.

Preferably these offretites have a secondary microporosity, measured bythe BJH method (defined in European patent 190 949) ranging from 3 to 5nm and corresponding to about 5-50% of the zeolite total pore volume.

Obviously, the modifying treatments used for offretite may be applied toany structure of the erionite-offretite type, particularly to ZSM-34zeolite and to AG-2, N-O, ZKU and T erionite zeolites.

The specific catalyst used according to the invention for increasing theoctane number of gasoline produced by cracking, contains:

(a) 20-95%, preferably 30-80% and more advantageously 50-80% by weightof at least one matrix (constituent A),

(b) 1-70%, preferably 4-60% and more advantageously 10-50% by weight ofat least one zeolite of open structure other than a zeolite of theerionite family (constituent B). It is recalled that the term zeolite ofopen structure, as used in the present invention, means a zeolite whosemain dodecagonal channels have an opening of such a size as to beequivalent to a circular opening of at least 7 Angstrom diameter,

(c) 0.05-40% preferably 0.1-30% and more advantageously 0.5-10% byweight of at least one zeolite of the erionite family (offretite,ZSM-34, AG2, N-O, ZKU or T erionite for example) having a potassiumcontent lower than 4% by weight, the total alkali metal content beingpreferably lower than 4% by weight (constituent C).

The sum of the percentages by weight of constituents A, B, C containedin the catalyst is always 100%.

When the zeolite of the erionite family is added to the main catalyst asspherical particles separate therefrom but of the same granulometry, theweight of zeolite of the erionite family ranges from 1 to 90%(preferably from 5 to 60%) of the weight of said particles (other thanthose of the main catalyst).

Constituents A and B have been above defined.

Constituent C of the specific catalyst according to the inventionpreferably mainly contains at least one offretite whose main dodecagonalchannels have an opening of less than 6.8 Angstroms (constituent C)whose SiO₂ /Al₂ O₃ molar ratio ranges from about 15 to about 500, thecrystalline parameters a and c of the elementary mesh being from 1.285to 1.315 nm for a and from about 0.748 to 0.757 nm for c, and thepotassium content lower than 1.5% by weight, the total alkali metalcontent being preferably lower than 1.5% by weight. All otherinformations concerning constituent C have been indicated above.

EXAMPLES

The following examples illustrate the invention without limiting thescope thereof.

EXAMPLE 1 (COMPARATIVE) Preparation of an offretite catalyst constituentnot conforming with the invention

200 g of synthetic offretite whose opening of the main dodecagonalchannels is 6.4 Angstroms (W. MEIER and D. H. OLSON, Atlas of ZeoliteStructure Types, 19781, having a SiO₂ /Al₂ O₃ molar ratio of 8,containing 9.9% by weight of potassium and 2.8% by weight oftetramethylammonium ions, have been roasted under a stream of a 80%nitrogen and 20% air mixture, flowing at a rate of 3 l/h/g, for 2 hoursat 500° C., in order to remove the TMA⁺ cations.

The obtained product (referenced P) has then been exchanged three timeswith a 2 M ammonium nitrate solution, at 100° C. for 4 hours, understirring, with a ratio of solution volume to the dry solid weight (V/P)equal to 5 cc g⁻¹.

The obtained solid, referred to as 1A, contains 2.8% by weight of sodiumand has a SiO₂ /Al₂ O₃ molar ratio of 8; its structural characteristicsare given in Table 2 hereinafter. Solid 1A is called product 1. Thediffraction diagram of product 1 (NH₄ ⁻ offretite) is reported in TableI below.

The opening of the main dodecagonal channels of offretite was notmodified by the treatments.

                  TABLE I    ______________________________________    X-ray diffraction diagram of product 1. (1A)    2 theta d (nm)   Intensity                              2 theta                                     d (nm) Intensity    ______________________________________    7.70    1.145    66       28.10  0.317   10    11.75   0.752     7       28.40  0.314   24    13.40   0.661    37       30.55  0.292    5    14.10   0.628     7       31.25  0.286   62    15.50   0.572    18       31.50  0.284   59    17.85   0.496     2       33.50  0.267   24    19.50   0.455    26    20.50   0.433    54    23.31   0.382    31    23.70   0.375    100    24.90   0.357    66    26.20   0.340    <1    27.00   0.330    23    27.30   0.327     7    ______________________________________

EXAMPLE 2 Preparation of an offretite catalyst constituent having a SiO₂/Al₂ O₃ molar ratio of 25, conforming with the invention (2F or product2)

Offretite 1A obtained in example 1, which contains 2.8% by weight ofpotassium, has been subjected to the following operations:

First cycle

Self-steaming at 550° C. for 2 hours (to obtain product 2A),

2 successive cation exchanges with NH₄ NO₃ 2 M at 100° C. for 4 hoursunder stirring, the ratio of the solution volume to the weight of drysolid (V/P) being 5 cc g⁻¹ (to obtain product 2B).

Second cycle

Self-steaming at 650° C. for 2 hours (to obtain product 2C),

cation exchange with NH4NO₃ 2 M in the same conditions as for obtaining2B (to obtain product 2D),

2 successive acid etchings by 0.23 N HCl, then by 0.36 N HCl at 100° C.for 4 hours, with a V/P ratio of 10 (to obtain product 2E after thefirst acid etching and product 2F after the second acid etching).

                                      TABLE 2    __________________________________________________________________________                 1A  2A  2B  2C  2D  2E  2F    __________________________________________________________________________    SiO.sub.2 /A1.sub.2 O.sub.3 (mol)                 8                   10  25    X-RAY DIFFRACTION    S lines (10.sup.3)                 277 303 323 298 274 290.5                                         285    S bottom (10.sup.3)                 203 193 209 240 256.5                                     269 256    S lines      58  61  61  55  52  52  53    (% S total)    Crystallinity                 100 105 105 95  90  90  91    Paramaters a 13.22                     13.10                         13.16                             13.02                                 13.02                                     13.01                                         13.03    (Angstroms) c                 7.52                     7.50                         7.51                             7.51                                 7.51                                     7.51                                         7.52    __________________________________________________________________________

At the end of these various treatments, the crystallinity of product 2Fis still excellent (its structural characteristics and its diffractiondiagram are reported in Tables 2 and 3), its potassium content is 0.7%by weight, its SiO₂ /Al₂ O₃ molar ratio is 25 and its water adsorptioncapacity is 15% (P/P_(o) =0.1). Solid 2F is called product 2. Theopening of the main dodecagonal channels was not modified by thetreatments. Its potassium content is 0.3% by weight; its wateradsorption capacity is 5% for a P/P_(o) ratio of 0.1. It has a secondarymicroporosity with diameters ranging from 20 to 60 Angstroms,corresponding to 0.10 cc/g for a total pore volume of 0.38 cc/g,measured with a P/P_(o) ratio of 0.9. This solid 3C is called product 3.The opening of the main dodecagonal channels was not modified by thetreatments.

                  TABLE 4    ______________________________________                 1 or    2 or    Products     1A      2F      3A    3B    3C    ______________________________________    SiO.sub.2 /Al.sub.2 O.sub.3 (mol.)                 8       25                  67.2    X-RAY DIFFRACTION    S lines (10.sup.3)                 277     285     306   316   330    S bottom (10.sup.3)                 203     256     248   255   259    S lines/S total (%)                 58      53      55    55    56    Cristallinity                 100     91      95    95    97    Parameters a 13.22   13.03   12.93 12.93 12.93    (Angstroms) c                 7.52    7.52    7.51  7.53  7.53    ______________________________________

                  TABLE 3    ______________________________________    X-RAY DIFFRACTION DIAGRAM OF PRODUCT 2    2 Theta d (nm)   Intensity                              2 theta                                     d (nm) Intensity    ______________________________________    7.85    1.13     80       24.0   0.371  44    11.80   0.751     7       24.95  0.356  47    13.60   0.650    100      27.40  0.325  27    14.15   0.625    43       28.50  0.313  20    15.70   0.564    33       30.95  0.289  14    18.00   0.492     2       31.70  0.282  71    19.70   0.451    16       33.90  0.264  22    20.85   0.426    57    23.70   0.375    29    ______________________________________

EXAMPLE 3 Preparation of an offretite catalyst constituent conformingwith the invention

The offretite 2F, called product 2, obtained in example 2, having asilica-to-alumina molar ratio of 25, has been subjected to the followingoperations:

self steaming at 750° C. for 2 hours, (product 3A),

cation exchange with NH₄ NO₃ 2 M in the conditions of example 1 (product3B),

acid etching with 0.61 N HCl (V/P=15) at 100° C. for 4 hours (product3C).

The obtained product 3C, whose SiO₂ /Al₂ O₃ molar ratio is 67, has anexcellent crystallinity (its structural characteristics and itsdiffraction diagram are reported in Tables 4 and 5).

                  TABLE 5    ______________________________________    X-RAY DIFRACTION DIAGRAM OF PRODUCT 3 (or 3C)    2 theta d nm     Intensity                              2 theta                                     d nm   Intensity    ______________________________________    7.90    1.12     100      31.85  0.281  40    11.80   0.751     5       34.10  0.2625 13    13.70   0.645    90    14.20   0.624    39    15.85   0.559    20    18.10   0.490     1    19.75   0.449    15    21.00   0.423    36    23.90   0.372    18    24.15   0.368    22    24.95   0.356    25    27.55   0.323    15    28.55   0.312     9    31.10   0.287     7    ______________________________________

EXAMPLE 4 Preparation of cracking additives essentially comprisingoffretite obtained according to the preceding examples

The various products 1 (or 1A), 2 (or 2F) and 3 (or 3C) are respectivelydiluted in a proportion of 30% by weight in an amorphous silica ofgranulometry similar to that of the used offretites.

The various obtained catalytic additives are pelletized, then reduced tosmall aggregates by means of a crushing machine. The 40-200 micronfraction is then recovered by screening. These additives arerespectively called 1, 2 and 3 offretite additives, in short A01, A02and A03.

EXAMPLE 5 (COMPARATIVE) Preparation of a cracking additive essentiallycomprising ZSM5 zeolite, called ZSM5 additive

A ZSM5 zeolite having a SiO₂ /Al₂ O₃ ratio of 60 has been synthesizedaccording to the technique disclosed by ARGAUER et al. in U.S. Pat. No.3,702,886. After controlled roasting, in air at 550° C., for removingthe totality of tetrapropylammonium ions, said zeolite is admixed withamorphous silica (identical to that used in example 4), in a proportionof 30% by weight of zeolite; then the resultant solid is pelletized,crushed and screened as precedingly (example 4). The obtained additiveis called ZSM5 additive, in short AZ4.

EXAMPLE 6 (COMPARATIVE) Preparation of a cracking additive essentiallycomprising mordenite zeolite, called mordenite additive

900 g of sodium mordenite (900 Na zeolon) as extrudates of 1.587 mm(1/16th inch) sold by U.S. Company NORTON, are subjected to two ionexchanges in an ammonium nitrate solution in the following conditions:

    ______________________________________    solution volume          3.6 liters    NO.sub.3 NH.sub.4 concentration of the solution                             2 M    exchange time            1 hour    exchange temperature     T = 90° C.    ______________________________________

After each exchange the solid is washed in 1.8 liter of distilled waterfor 20 minutes at 20° C.

At the end of these operations, the product is dried at 120° C. in astove for 3 hours. The obtained solid is roasted for 2 hours at 550° C.under a 180 liter per hour flow rate of moist air containing 50% byvolume of steam.

The mordenite thus obtained in hydrogen form is dipped into 9 liters ofa 0.6 M hydrochloric acid solution and is treated for 2 hours at 90° C.in said solution. After a 10 minute washing in 5.4 liters of distilledwater, followed with filtration, the product is dried at 120° C. for 2hours. The obtained mordenite is roasted in moist air in conditionssimilar to those above described, except for the temperature which isbrought to 600° C.

The obtained solid is dipped into 3.6 liters of a normal hydrochloricacid aqueous solution and is treated for 2 hours at 90° C. in saidsolution. After a 10 minute washing in 3.6 liters of distilled water at20° C., the solid is dried at 120° C. for 2 hours.

This solid is then subjected to roasting in moist air in conditionssimilar to those above described, except for the temperature which isbrought to 650° C.

The roasted solid is dipped into 1.8 liters of a 2 N hydrochloric acidaqueous solution and is treated for 2 hours at 90° C. in said solution.It is then washed in 5.4 liters of distilled water, filtered and driedat 120° C. for 2 hours.

The two essential characteristics of the obtained mordenite are:

total SiO₂ /Al₂ O₃ molar ratio=58.6

residual sodium % by weight=0.09

This product is diluted in a proportion of 30% by weight in amorphoussilica (identical to that used in example 4), then pelletized, crushedand screened as precedingly (example 4). The obtained additive is calledmordenite additive, in short AM5.

EXAMPLE 7 Preparation of cracking catalysts, comparative or conformingwith the invention, and test conditions for these catalysts

The various additives obtained in examples 4 to 6 are mechanically mixedconventionally in a proportion of 20% by weight with a fresh industrialcracking catalyst called in short CAT. This catalyst CAT contains asilica-alumina matrix and 30% by weight of ultra stabilized Y zeolite(USY) having a crystalline parameter of 2.45 nm, and whose maindodecagonal channels have an opening of 7.4 Angstroms (W. MEIER and D.HOLSON, Atlas of Zeolite Structure Types 1978). This catalyst (CAT) waspreviously roasted for 6 hours at 750° C. in the presence of 100% steam.

Each of the obtained catalysts is then introduced into the reactor of acatalyst test micro-unit MAT. The capacity of this catalyst to convert aheavy hydrocarbon charge is then determined in the following conditions:

    ______________________________________    catalyst amount          6 g    catalyst/charge weight ratio (C/O)                             6    charge injection time    40 seconds    weight hourly space velocity (WHSV)                             15 h.sup.-1    temperature              510° C.    ______________________________________

The treated charge has the following characteristics:

    ______________________________________    density at 15° C.                         0.929    S % by weight        0.155    N % by weight        0.1    Conradson carbon %   3.8    Ni + V               5 ppm    Viscosity at 60° C. (Cst)                         45.2    Refraction index R.sub.I                         1.5061    of the charge at 60°  C.    saturated            45.5% by weight    olefins              1.5% by weight    aromatics            42.7% by weight    resins               9.8% by weight    asphaltenes (insoluble to C.sub.5)                         3.1% by weight    ______________________________________

Comparison of the catalytic performances

The results are expressed as follows:

charge conversion in % by weight

gas yield in % by weight (H₂ +C₁ -C₄ hydrocarbons)

gas distribution in % by weight (H₂ +C₁ -C₄ hydrocarbons)

gasoline yield in C₅ % by weight at 220° C.

LCO yield (middle distillates: 220°-380° C.) in % by weight

coke yield in % by weight

The results obtained by the various catalysts and the industrialcracking catalysts are summarized in Table 6 hereinafter.

                                      TABLE 6    __________________________________________________________________________                             CAT + CAT +                             AZ4*  AMS*                             essen-                                   essen-                CAT CAT  CAT tially                                   tially                +   +    +   of    of    CATALYSTS            CAT*                AO1*                    AO2  AO3 ZSM5  mordenite    __________________________________________________________________________    Conversion            67.7                67.3                    69.4 69.8                             65.7  65.0    Total gas            19.4                20.4                    23.9 25.0                             20.3  20.1    Gasoline            40.5                39.1                    37.6 37.2                             37.9  37.0    LCO     20.7                19.0                    18.9 18.8                             20.1  20.7    Coke    7.8 7.8 7.9  7.6 7.5   7.9    H.sub.2 0.06                0.05                    0.05 0.05                             0.06  0.06    C.sub.1 + C.sub.2            1.7 1.7 1.8  1.6 1.9   1.7    C.sub.3 saturated            1.4 1.8 2.7  2.8 2.1   1.7    C.sub.3 unsaturated            5.2 6.0 7.9  9.1 6.1   5.9    C.sub.4 saturated            5.3 5.2 5.6  5.5 4.9   5.1    C.sub.4 unsaturated            5.7 5.6 5.9  5.9 5.3   5.7    __________________________________________________________________________     *comparison examples

These results show that the catalysts containing A02 and A03 offretiteadditives prepared from dealuminated offretites having respective SiO₂/Al₂ O₃ ratios of 25 and 67 as well as the other characteristics of theinvention, provide clearly improved propylene yields as compared with aconventional cracking catalyst CAT and with catalysts containing AZ5additives including ZSM5 or AM5 zeolite containing a mordenite notconforming with the invention.

EXAMPLE 8 Preparation of offretite OFF1 having a SiO₂ /Al₂ O₃ molarratio of 52

The solid 1A, not conforming with the invention, as prepared in example1, is subjected to the following successive treatments:

self-steaming at 550° C. for 2 hours (to obtain product 2A),

two successive cation exchanges in a 2 M solution of NH₄ NO₃, at 100° C.for 4 hours, under stirring, with a ratio of solution volume to soliddry weight (V/P) equal to 5 cc.g⁻¹ (to obtain product 2B),

self-steaming at 650° C. for 2 hours (to obtain product 2C),

2 successive cation exchanges in a 2 M solution of NH₄ NO₃, at 100° C.for 4 hours, under stirring, with a ratio of solution volume to soliddry weight (V/P) equal to 5 cc.g⁻¹ (to obtain product 2D),

acid etching in a 1 N solution of HCl at 100° C. for 2 hours with aratio of solution volume to the weight of dry solid equal to 15 cc.g⁻¹.

The obtained solid is referred to as OFF1. It has the followingcharacteristics:

    ______________________________________    SiO.sub.2 /Al.sub.2 O.sub.3 ratio (mole)                            52    % K                     0.13 by weight    Crystalline parameters      a                     13.01 Angstroms      c                     7.53 Angstroms    Cristallinity (DX)      91%    Micropore volume N.sub.2                            0.28 cc.g.sup.-1    Water adsorption capacity (by weight)                            3%    Cyclohexane adsorption capacity (by weight)                            7%    ______________________________________

As indicated by the preceding characteristics, solid OFF1 is adealuminated offretite of SiO₂ /Al₂ O₃ ratio equal to 52, wellcrystallized and having a very low potassium residual content and a highmicropore volume.

EXAMPLE 9 Preparation of offretite OFF2 having a SiO₂ /Al₂ O₃ molarratio of 105

The product referenced OFF1, as obtained in example 8, is subjected tothe following treatments:

self-steaming at 750° C. for 2 hours,

acid etching in 3 N HCl at 100° C., for 4 hours, with a ratio ofsolution volume to dry solid weight (V/P) of 15 cc.g⁻¹.

The obtained solid, referenced OFF2, has the following characteristics:

    ______________________________________    SiO.sub.2 /Al.sub.2 O.sub.3                            103    K (by weight)           0.04%    Crystalline parameters      a                     12.97 Angstroms      c                     7.54 Angstroms    Crystallinity (DX)      87%    Micropore volume N.sub.2                            0.27 cc.g.sup.-1    Water adsorption capacity (by weight)                            1.1%    Cyclohexane adsorption capacity (by weight)                            6.8%    ______________________________________

As solid OFF1, solid OFF2 is a much dealuminated offretite of SiO₂ /Al₂O₃ molar ratio equal to 105, well crystallized, of low potassiumresidual content and of high micropore volume.

EXAMPLE 10 (NOT CONFORMING WITH THE INVENTION) Cracking performed over areference industrial balanced catalyst (CAT E)

A catalytic cracking test is performed with a hydrocarbon charge whosecharacteristics are given in table 7. The used industrial balancedcatalyst (called CAT E) contains 70% of a conventional matrix,essentially formed of alumina of high silica content and of kaolin, andcontaining 30% of USY ultrastable Y zeolite. It has the followingcharacteristics:

    ______________________________________    surface in m.sup.2.g.sup.-1                            110    rare-earth oxide in % by weight                            1.6.    Na.sub.2 O (% by weight)                            0.3    V (ppm)                 4800    Ni (ppm)                2800    Fe (ppm)                10200    ______________________________________

This catalyst CAT E is placed into a reactor of a test micro-unit (alsocalled micro-activity test unit, in short MAT) and contacted with thehydrocarbon charge in the following test conditions:

    ______________________________________    T                    520° C.    C/O                  5-6.5 (*)    catalyst amount      5 g    injection time       40 seconds    ______________________________________     (*) In all the following examples, the C/O ratio will be calculated by     taking only into account the reference catalyst, irrespective of the     eventual presence of additive (mainly consisting of offretite). In other     terms, C = weight in grams of the reference catalyst CAT E.

The Re search and Motor octane numbers of light of heavy gasolines arecalculated from detailed chromatography analyses of the C₅ ⁺ liquideffluent recovered at the reactor output at the end of the catalysttest.

The results are reported in Table 8 hereinafter.

                  TABLE 7    ______________________________________    CHARACTERISTICS OF THE HEAVY CHARGE    ______________________________________    Density (20° C.)                         0.968    Viscosity (solid at 60° C.)    (80° C.)      119.8    cSt (100° C.) 52.2    Conradson (% by weight)                         5.1    Na ppm               2    Ni ppm               12    V ppm                1    C (% by weight)      86.9    H (% by weight)      12.2    N (% by weight)      0.35    S (% by weight)      0.21    N basic (% by weight)                         0.055    C aromatic (% by weight)                         22.3    H aromatic (% by weight)                         2.7    Simulated Distillation (°C.)    5% by weight         367    10% by weight        399    20% by weight        436    40% by weight        495    60% by weight        575    Final boiling point  575    ______________________________________

                  TABLE 8    ______________________________________    Catalyst        CAT E    Test N°  1          2      3    ______________________________________    C/O* (% by weight)                    5.0        5.5    6.5    Conversion      62.36      69.69  73.60    C.sub.1 -C.sub.4                    12.75      16.63  17.98    (C.sub.5 -150) Gasoline                    30.22      33.39  32.93    (150-221) Gasoline                    11.13      11.63  10.49    Total Gasoline  41.35      45.02  43.42    LCO (221-350)   15.68      14.45  12.50    Slurry (350.sup.+)                    21.97      15.86  13.90    Coke            7.75       7.54   11.67    H.sub.2         0.51       0.50   0.54    C.sub.1         0.57       0.66   0.68    C.sub.2         0.45       0.47   0.49    C.sub.2.sup. =  0.60       0.77   0.84    Total C.sub.2   1.05       1.24   1.33    C.sub.3         0.63       1.02   1.18    C.sub.3 .sup.=  3.20       3.82   4.10    Total C.sub.3   3.83       4.84   5.28    iC.sub.4        1.56       2.92   3.36    nC.sub.4        0.49       0.85   0.98    iC.sub.4.sup. = 1.77       1.94   2.03    nC.sub.4.sup. = 3.47       4.17   4.32    Total C.sub.4   7.29       9.88   10.69    light gasoline RON                    92.4       92.8   93.3    heavy gasoline RON                    88.6       88.9   89.4    light gasoline MON                    77.9       78.3   78.5    heavy gasoline MON                    79.2       79.5   79.9    ______________________________________     *in C/O, C = weight of catalyst CAT E.

in test 4 of example 12 (table 9), the octane number and gasoline yielddifferences with respect to test 1 of the preceding example 8, are:

    ______________________________________    Gasoline yield  -3.89    Light gasoline RON                    +1.8    Heavy gasoline RON                    +8.9    Light gasoline MON                    +3.6    Heavy gasoline MON                    +5.2    Propylene yield +3.31 (i.e. 3.31/3.20 = 103%)    Isobutane yield +2.40 (i.e. 2.40/1.56 = 154%)    Yield of n butenes                    +2.0 (i.e. 2/3.47 = 58%)    ______________________________________

These examples show that the addition of small offretite amounts to theconventional cracking catalyst clearly improves the gasoline yield andthe propylene and isobutane production, and, to a less extent, theproduction of normal butenes.

EXAMPLE 13 (CONFORMING WITH THE INVENTION)

A new catalyst consisting of the following mixture of preceding catalystCAT E and OFF2 offretite-containing additive (SiO₂ /Al₂ O₃ =105) issurveyed in the same conditions and with the same charge as precedingly(examples 3 and 5):

CAT E+5% by weight of OFF2

                  TABLE 9    ______________________________________               CAT E                 2% by    CATALYST     weight OFF1 5% by weight OFF1    ______________________________________    Test N°                 4       5       6     7     8    C/O (*)      5.0     5.4     4     4.8   5.5    (% by weight)    Conversion   68.89   71.36   60.99 68.47 73.10    C.sub.1 -C.sub.4                 23.29   26.08   23.91 27.62 33.26    (C.sub.5 -150) Gasoline                 28.41   27.56   24.01 24.63 23.07    (150-221) Gasoline                 9.04    9.00    6.26  7.03  6.19    Total Gasoline                 37.46   36.56   30.27 31.66 29.27    LCO (221-350)                 13.38   12.01   11.26 11.90 10.87    Slurry (350.sup.+)                 17.73   16.18   27.76 19.64 16.03    Coke         7.75    8.73    6.47  8.81  10.11    H.sub.2      0.39    0.45    0.34  0.39  0.47    C.sub.1      0.54    0.57    0.43  0.56  0.56    C.sub.2      0.40    0.41    0.35  0.41  0.45    C.sub.2.sup. =                 1.04    1.23    0.99  1.48  2.00    Total C.sub.2                 1.44    1.64    1.35  1.88  2.46    C.sub.3      1.48    1.96    1.46  2.11  3.37    C.sub.3.sup. =                 6.51    7.21    6.20  7.37  8.53    Total C.sub.3                 7.99    9.17    7.65  9.48  11.90    iC.sub.4     3.96    4.54    3.33  4.79  5.80    nC.sub.4     1.12    1.32    1.06  1.41  2.04    iC.sub.4.sup. =                 2.78    3.02    4.25  3.78  4.16    nC.sub.4.sup. =                 5.46    5.82    5.84  5.72  6.35    Total C.sub.4                 13.32   14.70   14.48 15.70 18.35    Light gasoline RON                 94.2    95.6    96.0  97.5  98.5    Heavy gasoline RON                 97.5    98.2    93.4  98.8  99.1    Light gasoline MON                 81.5    82.1    80.8  82.3  82.1    Heavy gasoline MON                 84.4    85.5    82.2  85.7  85.6    ______________________________________     *in C/O, C = weight of catalyst CAT E.

The results are reported in Table 10 below.

As compared with the preceding example 11, the use of highlydealuminated OFF2 offretite having a Si/Al atomic ratio of 52.5, muchhigher than that OFF1 offretite, results in a lower activity (conversionrate of 65.49 instead of about 68.5-69), in a lower propylene production(4.79 instead of about 6.5-7%), in a lower isobutane production (3.14instead of about 4-4.5), in a higher gasoline production but with lowerResearch and Motor octane numbers. Nonetheless, as compared with thoseof example 10, these results clearly show the effect of said offretitefor improving the production of propylene, isobutane, and butenes andfor improving the Research and Motor gasoline octane numbers.

It is again recalled that, for these results, the value C of the C/Oratio is equal to the weight of the reference catalyst CAT E.

                  TABLE 10    ______________________________________    Catalyst         Cat E + 5% OFF2    ______________________________________    Test N°   9    C/O (*)          4.9    (% by weight)    Conversion       65.49    C.sub.1 -C.sub.4 18.93    (C.sub.5 -150) Gasoline                     28.92    (150-221) Gasoline                     9.72    Total Gasoline   38.64    LCO (221-350     13.91    Slurry (350.sup.+)                     20.37    Coke             7.92    H.sub.2          0.51    C.sub.1          0.65    C.sub.2          0.49    C.sub.2.sup. =   1.09    Total C.sub.2    1.58    C.sub.3          1.09    C.sub.3.sup. =   4.79    Total C.sub.3    5.88    iC.sub.4         3.14    nC.sub.4         0.87    iC.sub.4.sup. =  2.12    nC.sub.4.sup. =  4.69    Total C.sub.4    10.82    Light Gasoline RON                     94.1    Heavy Gasoline RON                     93.8    Light Gasoline MON                     79.9    Heavy Gasoline MON                     82.1    ______________________________________     + in C/O, C = weight of catalyst CAT E.

What is claimed as the invention is:
 1. In a process for catalyticcracking of a hydrocarbon charge, comprising subjecting said charge tocracking conditions in the presence of a catalyst, the improvementwherein the catalyst contains:(a) 20-95% by weight of at least onematrix, selected from the group consisting of alumina, silica, magnesia,clay, titanium oxide, zirconia, combinations of at least two of thesecompounds and alumina-boron oxide combinations, (b) 1-60% by weight ofat least one zeolite of open structure whose main dodecagonal channelshave openings of at least 7 Angstroms, said zeolite of open structurebeing selected from the group formed of X, Y, L, omega and betazeolites, (c) 0.5-60% by weight of at least one dealuminated offretitewhose main dodecagonal channels have openings smaller than 7 Angstromsand which has a SiO₂ /Al₂ O₃ molar ratio from about 15 to about 500, thecrystalline parameters a and c of elementary mesh ranging respectivelyfrom 1.285 to 1.315 nm for a and from 0.748 to 0.757 nm for c and has apotassium content lower than 1.5% by weight, and is prepared by aprocess comprising:(1) preparing a mixture of offretite with a matrix,(2) preparing a mixture of a zeolite of open structure with a matrix,and (3) admixing the product of (1) with the product of (2).
 2. Aprocess according to claim 1, wherein the catalyst contains:(a) 30-80%by weight of matrix, (b) 4-50% by weight of zeolite of open structure,(c) 1-40% by weight of offretite.
 3. A process according to claim 2,wherein in the catalyst the zeolite of open structure is: (a) anultrastable Y zeolite in hydrogen form, (b) the latter zeolite at leastpartially exchanged with cations of metals from the rare-earth grouphaving an atomic number from 57 to 71 inclusive, of (c) a mixturethereof.
 4. A process according to claim 1, wherein the offretite has aSiO₂ /Al₂ O₃ molar ratio from about 20 to about 400, crystallineparameters a and c of elementary mesh respectively from about 1.290 nmto about 1.310 nm for a and from about 0.750 nm to about 0.755 nm for cand a potassium content lower than 1% by weight.
 5. A process accordingto claim 4, wherein the offretite is in hydrogen form.
 6. A processaccording to claim 4, wherein the offretite is at least partiallyexchanged with cations of metals from the rare-earth group of atomicnumber from 57 to 71 inclusive.
 7. A process according to claim 1,wherein said constituent (c) further contains a minor portion oferionite, ZSM-34, AG2, N-O, ZKU or erionite zeolites.
 8. A processaccording to claim 7, wherein said constituent (c) of the catalystessentially consists of at least one offretite whose main dodecagonalchannels have openings smaller than 6.8 Angstroms.
 9. A processaccording to claim 8, wherein the offretite has a nitrogen adsorptioncapability higher than 0.2 cc of liquid per gram, a cyclohexaneadsorption capacity higher than 4% by weight and a water adsorptioncapacity lower than about 10% by weight.
 10. A process according toclaim 1, having a benzene adsorption capacity at 25° C., under a partialpressure of 70 torr higher than 5% by weight, and a second microporosityof 3-5 nm and corresponding to about 5-50% of the zeolite total porevolume.
 11. A process according to claim 1, wherein in the catalystcomponent (c) having a SiO₂ /Al₂ O₃ ratio of 30-300, a crystallineparameter a of about 1.294 to 1.300 nm, a crystalline parameter c ofabout 0.750-0.755 nm, and a potassium content lower than about 0.5% byweight.
 12. A process according to claim 11, having a benzene adsorptioncapacity at 25° C. at a partial pressure of 70 torr higher than about 6%by weight, a water adsorption capacity at 25° C. for a P/P_(o) ratio of0.1, lower than 13% by weight, and a secondary microporosity of 3-5 nmcorresponding to 5-50% of the zeolite total pore volume.